|Fig. 16-5 Simplified control system.|
|Fig. 16-6 A simplified typical afterburner fuel control system|
17. When afterburning is selected, a signal is relayed to the afterburner fuel control unit. The unit determines the total fuel delivery of the pump and controls the distribution of fuel flow to the burner assembly. Fuel from the burners is ignited, resulting in an increase in jet pipe pressure (P6). This alters the pressure ratio across the turbine (P3/P6), and the exit area of the jet pipe nozzle is automatically increased until the correct PS/PS ratio has been restored. With a further increase in the degree of afterburning, the nozzle area is progressively increased to maintain a satisfactory P3/P6 ratio. Fig. 16-6 illustrates a typical afterburner fuel control system.
|Fig. 16-7 A simplified typical afterburner nozzle control system.|
18. To operate the propelling nozzle against the large 'drag' loads imposed by the gas stream, a pump and either hydraulically or pneumatically operated rams are incorporated in the control system. The system shown in fig. 16-7 uses oil as the hydraulic medium, but some systems use fuel. Nozzle movement is achieved by the hydraulic operating rams which are pressurized by an oil pump, pump output being controlled by a linkage from the pressure ratio control unit. When an increase in afterburning is selected, the afterburner fuel control unit schedules an increase in fuel pump output. The jet pipe pressure (P6) increases, altering the pressure ratio across the turbine (P3/P6). The pressure ratio control unit alters oil pump output, causing an out-of-balance condition between the hydraulic ram load and the gas load on the nozzle flaps. The gas load opens the nozzle to increase its exit area and, as the nozzle opens, the increase in nozzle area restores the P3/P6 ratio and the pressure ratio control unit alters oil pump output until balance is restored between the hydraulic rams and the gas loading on the nozzle flaps.